This invention relates to a liquid-crystal display device, and more particularly to a liquid-crystal display device capable of improving display quality.
In recent years, liquid-crystal display devices, active-matrix liquid-crystal optoelectronic elements using thin-film transistors, have been widely used. Although these liquid-crystal display devices have a relatively small screen size with a diagonal length of up to about 50 cm, they have been used as display elements in notebook personal computers, personal digital assistants, pocket-sized televisions because of light weight, thinness, and low power consumption. As for display devices having a large screen with a diagonal length of up to about 250 cm, projection display devices using the liquid-crystal display devices as light bulbs have been used.
A color-display active-matrix liquid-crystal display widely used at present is such that a liquid-crystal composition is sandwiched between two substrates having electrodes, the periphery of the pair of substrates excluding the liquid crystal injection opening is fixed with adhesive, and then the liquid crystal injection opening is sealed with sealer. One substrate, or the array substrate, includes a plurality of scanning lines and signal lines arranged so as to cross at right angles, thin-film transistors using amorphous silicon (a-Si) as semiconductor layers arranged near the intersections of the scanning lines and signal lines, and pixel electrodes connected to the thin-film transistors. The other substrate, or the counter substrate, includes the counter electrodes facing the plurality of pixel electrodes and a color filter layer composed of colored layers of the three primary colors (red (R), green (G), and blue (B)). The liquid-crystal display device is, for example, of the TN type, STN type, GH type, or ECB type. Alternatively, it may be composed of ferroelectric liquid crystal. On both sides of the pair of substrates, polarizing plates are provided. At the back of the array substrate, a backlight unit is provided.
One of the problems regarding the performance of such a liquid-crystal display device is an improvement in transmittance. In the above-mentioned active matrix liquid-crystal display device, to improve transmittance, the aperture ratio has only to be improved. To do this, the size of the pixel electrode of each pixel is made greater. Since the scanning lines and signal lines are provided around the pixel electrodes, each pixel electrode cannot be made wider without any restriction. A known pixel overlying structure to avoid this problem is such that an insulating film is provided between the scanning lines and signal lines and the pixel electrodes on the array substrate, thereby allowing the pixel electrodes to overlap with the signal lines or scanning lines. Since the signal lines and scanning lines are made of shading metal in a liquid-crystal display device with the pixel overlying structure, they act as a black matrix that shields one pixel electrode from another. This makes it unnecessary to form a black matrix on the counter substrate side.
When the pixel overlying structure liquid-crystal device is viewed obliquely, however, surface irregularities are seen, causing the problem of decreasing display quality. The result of analyzing this problem showed that, when the liquid-crystal display device was viewed at an angle of inclination larger than a specific value, part of the light passing through around the pixel electrodes was seen. Furthermore, since the amount of overlap of the pixel electrodes with the signal lines or scanning lines varied subtly, the amount of light passing through around the pixel electrodes varied. Thus, the result also showed that the variations in the amount of the passing light were recognized as display irregularities.
It is, accordingly, an object of the present invention to provide a liquid-crystal display device capable of alleviating a decrease in display quality even when the display device is viewed obliquely.
The foregoing object is accomplished by providing a liquid-crystal display device comprising: a liquid-crystal display panel with a liquid-crystal composition sandwiched between first and second substrates forming a pair; and a backlight unit for illuminating the liquid-crystal display panel from behind, wherein the first substrate includes a wiring section, switching elements, an insulating film formed so as to cover the wiring section and the switching elements, and pixel electrodes formed in a matrix on the insulating film and connected to the switching elements through contact holes made in the insulating film, and where the thickness of the insulating film provided between the wiring section and the pixel electrodes is c, the refractive index of the insulating film is nins, the thickness of the liquid-crystal composition layer is d, the refractive index of the liquid-crystal composition is nLC, and the angle at which the luminance in the direction of azimuth of the backlight unit becomes half of the front luminance is xcex8, the amount a of overlap of the wiring section with the pixel electrodes is expressed as:
axe2x89xa7cxc2x7tan[sinxe2x88x921{(1/nins)sin xcex8}]
+(xc2xd)dxc2x7tan[sinxe2x88x921{(1/nLC)sin xcex8}]
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.